A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The substrate is clamped onto a substrate holder in the lithographic apparatus when transferring a pattern from the patterning device. A substrate holder conventionally has a plurality of burls to support the substrate. The total area of the burls that contacts the substrate is small compared to the total area of a substrate. Therefore, the chance that a contaminant particle randomly located on the surface of the substrate or the substrate holder is trapped between a burl and the substrate is small. Also, in manufacture of the substrate holder, the tops of the burls can be made more accurately coplanar than a large surface can be made accurately flat.
When a substrate is first loaded onto the substrate holder in preparation for exposure, the substrate is supported by so-called e-pins which hold the substrate at three positions. While the substrate is being held by the e-pins, its own weight will cause the substrate to distort, e.g. becoming convex when viewed from above. To load the substrate onto the substrate holder, the e-pins are retracted so that the substrate is supported by burls of the substrate holder. As the substrate is lowered onto the burls of the substrate holder, the substrate will contact in some places, e.g. near the edge, before other places, e.g. near the center. Friction between the burls and the lower surface of the substrate may prevent the substrate from fully relaxing into a flat unstressed state.
Curvature caused by the weight of the substrate when supported on the e-pins is relatively small, due to the rigidity of the substrate. Additionally, some relaxation does occur when the substrate is on the burls of the substrate holder. Nevertheless, the residual curvature can be sufficient to cause undesirable overlay errors. Furthermore, it is possible for the substrate to be non-flat (e.g. warped) even before being supported on the e-pins, which will increase errors.